In recent years, miniaturization of a digital still camera and a video camera having an optical zoom capability has been promoted. Thus, miniaturization and thinner design of image pickup apparatuses to be installed into those apparatuses have been required. Demand for a compact image pickup apparatus, which is capable of being configured into a cellular phone and a portable information terminal, has increased. For this demand, a folded optical system and a retractable optical system have been proposed. From the viewpoint of shortening the time to start photographing, the folded optical system is superior to the retractable optical system. As a technology in which a relationship between a diaphragm and a shutter is defined in order to further miniaturize the folded optical system, US2002/0285970 has disclosed a system where a diaphragm and a shutter are arranged to be separate pieces and U.S. Pat. No. 6,870,691 has disclosed a method of not to move a shutter when varying an image-pickup magnification.
However, since the optical system disclosed in US2002/0285970 includes a shutter movably arranged between a negative first lens group and a positive second lens group, the shutter can be miniaturized. However, the distance between the first lens group and the second lens group needs to be secured at the telephoto end of the optical system. Thus it is hard to make the variable power ratio large while keeping the same total length.
Further, since the optical system described in U.S. Pat. No. 6,870,691 includes the shutter whose position is fixed, an axial light flux and an off-axial light flux at the position of the shutter change largely depending on the zooming position. Thus, when in a high-speed shutter operation, unevenness of luminance tends to occur. A shading correction needs to be conducted depending on the degree of the unevenness of luminance.